Identify the formula for net force.

According to Newton's second law of motion, the net force ($F$) acting on an object is equal to the product of its mass ($m$) and acceleration ($a$).

Given mass $m = 1200 \text{ kg}$ and acceleration $a = 3 \text{ m/s}^2$.

The magnitude of the net force is $\boxed{\text{3600 N}}$.

4) (ب) (2)

For the seesaw to be balanced, the sum of clockwise moments must equal the sum of counter-clockwise moments about the pivot.
Given:
Distance of boy from pivot ($d_{boy}$) = 2 m
Distance of man from pivot ($d_{man}$) = 1.5 m
Weight of man ($W_{man}$) = 600 N
Acceleration due to gravity ($g$) = 10 m/s²

The man creates a clockwise moment.

For equilibrium, the boy's counter-clockwise moment must equal the man's clockwise moment.

Weight is related to mass and gravity by the formula $W = mg$.

The mass of the boy is $\boxed{\text{45 kg}}$.